grep: add --heading

With --heading, the filename is printed once before matches from that
file instead of at the start of each line, giving more screen space to
the actual search results.

This option is taken from ack (http://betterthangrep.com/).  And now
git grep can dress up like it:

	$ git config alias.ack "grep --break --heading --line-number"

	$ git ack -e --heading
	Documentation/git-grep.txt
	154:--heading::

	t/t7810-grep.sh
	785:test_expect_success 'grep --heading' '
	786:    git grep --heading -e char -e lo_w hello.c hello_world >actual &&
	808:    git grep --break --heading -n --color \

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -151,6 +151,10 @@ OPTIONS
 --break::
 	Print an empty line between matches from different files.
 
+--heading::
+	Show the filename above the matches in that file instead of
+	at the start of each shown line.
+
 -[ABC] <context>::
 	Show `context` trailing (`A` -- after), or leading (`B`
 	-- before), or both (`C` -- context) lines, and place a

builtin/grep.c

@@ -824,6 +824,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		OPT__COLOR(&opt.color, "highlight matches"),
 		OPT_BOOLEAN(0, "break", &opt.file_break,
 			"print empty line between matches from different files"),
+		OPT_BOOLEAN(0, "heading", &opt.heading,
+			"show filename only once above matches from same file"),
 		OPT_GROUP(""),
 		OPT_CALLBACK('C', NULL, &opt, "n",
 			"show <n> context lines before and after matches",

grep.c

@@ -735,9 +735,13 @@ static void show_line(struct grep_opt *opt, char *bol, char *eol,
 			opt->output(opt, "\n", 1);
 		}
 	}
+	if (opt->heading && opt->last_shown == 0) {
+		output_color(opt, name, strlen(name), opt->color_filename);
+		opt->output(opt, "\n", 1);
+	}
 	opt->last_shown = lno;
 
-	if (opt->pathname) {
+	if (!opt->heading && opt->pathname) {
 		output_color(opt, name, strlen(name), opt->color_filename);
 		output_sep(opt, sign);
 	}

grep.h

@@ -111,6 +111,7 @@ struct grep_opt {
 	unsigned last_shown;
 	int show_hunk_mark;
 	int file_break;
+	int heading;
 	void *priv;
 
 	void (*output)(struct grep_opt *opt, const void *data, size_t size);

t/t7810-grep.sh

@@ -774,4 +774,41 @@ test_expect_success 'grep --break with context' '
 	test_cmp expected actual
 '
 
+cat >expected <<EOF
+hello.c
+int main(int argc, const char **argv)
+	/* char ?? */
+hello_world
+Hello_world
+EOF
+
+test_expect_success 'grep --heading' '
+	git grep --heading -e char -e lo_w hello.c hello_world >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
+<BOLD;GREEN>hello.c<RESET>
+2:int main(int argc, const <BLACK;BYELLOW>char<RESET> **argv)
+6:	/* <BLACK;BYELLOW>char<RESET> ?? */
+
+<BOLD;GREEN>hello_world<RESET>
+3:Hel<BLACK;BYELLOW>lo_w<RESET>orld
+EOF
+
+test_expect_success 'mimic ack-grep --group' '
+	test_config color.grep.context		normal &&
+	test_config color.grep.filename		"bold green" &&
+	test_config color.grep.function		normal &&
+	test_config color.grep.linenumber	normal &&
+	test_config color.grep.match		"black yellow" &&
+	test_config color.grep.selected		normal &&
+	test_config color.grep.separator	normal &&
+
+	git grep --break --heading -n --color \
+		-e char -e lo_w hello.c hello_world |
+	test_decode_color >actual &&
+	test_cmp expected actual
+'
+
 test_done